1. Field of the Invention
The present invention relates to surface acoustic wave sensors such as biosensors and gas sensors. The present invention particularly relates to a surface acoustic wave sensor, using an SH-type surface acoustic wave, for detecting a target substance on the basis of a change in frequency due to the mass applied to the sensor.
2. Description of the Related Art
Various surface acoustic wave sensors including surface acoustic wave elements have been used to detect various substances. For example, a surface acoustic wave sensor for detecting biological substances such as DNAs and antibodies includes a reaction membrane, placed on a surface acoustic wave element, reacting only with a specific biological substance such as a DNA or an antibody. Such a DNA or antibody is bound to the reaction membrane by reaction. As a result, a mass is applied to the surface acoustic wave element. The presence or content of the DNA or the antibody is detected on the basis of the change in frequency due to the applied mass.
Japanese Unexamined Patent Application Publication No. 10-90270 discloses an example of this type of surface acoustic wave sensor. The surface acoustic wave sensor disclosed in this known document can detect 2-MIB (2-methylisoborneol) that is an earthy-smelling substance contained in water. With reference to FIG. 12, the surface acoustic wave sensor 101 includes interdigital transducers 103 and 104, a metal thin-film 105, and a piezoelectric substrate 102 on which the interdigital transducers 103 and 104 and the metal thin-film 105 are arranged. Amplifiers 106 and 107 are placed between one pair of the interdigital transducers 103 and the other pair of the interdigital transducers 104 and connected thereto. A mixer 108 is connected to rear portions of the interdigital transducers 104 for output and rear portions of the amplifiers 106 and 107. An output from the surface acoustic wave sensor 101 is extracted from the mixer 108.
In the surface acoustic wave sensor 101, a camphor-Ova complex is immobilized on the upper surface of the piezoelectric substrate 102. The camphor-Ova complex functions as a reaction membrane. As a result, 2-MIB is detected by the reaction between the camphor-Ova complex and 2-MIB.
That is, a complex antigen containing protein and camphor having a structure similar to that of 2-MIB, which is an earthy-smelling substance, is immobilized on the surface acoustic wave sensor 101. The surface acoustic wave sensor 101 is immersed in a test solution containing a certain amount of a 2-MIB antibody specifically bound to 2-MIB and the camphor-protein complex antigen competitively reacts with 2-MIB which is present in the solution and of which the content is unknown. The amount of the 2-MIB antibody bound to the camphor-protein complex antigen immobilized on the surface acoustic wave sensor 101 is determined from the change in output due to the mass applied to the surface acoustic wave sensor. The content of 2-MIB in the test solution is determined from the difference between the amount of the 2-MIB antibody bound to the camphor-protein complex antigen in the presence of 2-MIB and the amount of that in the absence of 2-MIB.
As described above, surface acoustic wave sensors have been widely used to detect or determine biological substances such as DNAs, antigens, and antibodies and various substances such as 2-MIB, which is a cause of earthy odor. This type of surface acoustic wave sensor includes a piezoelectric substrate and a reaction membrane, placed thereon, which is suitable for a target substance. The target substance is detected or determined from the change in frequency due to the mass applied to the reaction membrane.
In the surface acoustic wave sensor, the change in mass is detected as the change in frequency. Therefore, an increase in frequency change leads to an increase in the sensitivity of the surface acoustic wave sensor. In order to enhance the sensitivity, reaction membranes suitable for target substances have been widely investigated.
On the other hand, the relationship between the sensitivity of the surface acoustic wave sensor and the structure of a surface acoustic wave element included in the surface acoustic wave sensor has not been intensively investigated.